1. Field
The present disclosure relates to an induction motor and a control method thereof, and more particularly, to a single external power supply variable speed induction motor and a control method thereof.
2. Description of the Related Art
A general induction motor is an alternating current motor which is directly connected to a 3-phase power supply to be started by direct online. However, when an inducer is directly operated, an inrush current is large. Accordingly, in a case of a large capacity induction motor, the direct online is difficult, and a speed control is also difficult. For the speed control and the initial start, an inverter has to be used in the induction motor. However, when the inverter is used, there is a defect of causing harmonics of a systematic power supply on characteristics of the inverter. To supplementing this, it is general to connect the inverter to a converter. FIG. 1 shows a direct online start driving system of a general squirrel cage induction motor. Since a power conversion device is not used between a 3-phase systematic power supply and an induction motor, it is possible to drive the motor with the lowest cost. However, there is a defect that it is difficult to control a speed of the motor, and overcurrent flows at the time of initial start of the motor.
To control the speed of the induction motor, generally, an inverter is used. An alternating current voltage source necessary for the inverter may be configured by using a low-cost diode rectifier. FIG. 2 shows a driving system using such a diode rectifier. In such a case, although an induction motor is driven at a variable speed, there a defect that harmonics drastically occurs in a systematic power supply due to the diode rectifier.
To satisfy harmonic standard of the 3-phase power supply while driving the induction motor at the variable speed, a structure of attaching a PWM converter between the inverter and the systematic power supply may be used. This is shown in FIG. 3. In such a configuration, it is possible to lower a harmonic ratio of the systematic power supply while driving the induction motor at the variable speed. However, the system with such a structure requires a high cost.
A doubly feed driving system with a structure similar to the structure shown in FIG. 3 may be used as shown in FIG. 4. In the structure, a stator is directly connected to a 3-phase power supply, and rotor coils are connected to the systematic power supply through a separate power conversion device. In this case, the power conversion device connected to the rotor has to enable the power to flow in two ways, and thus the converter has to be used with the inverter. To connect the wires of the rotor to the power conversion device, an additional connection structure such as a slip-ring is necessary for the motor. Korean Patent Publication No. 1998-087820 discloses a multi-slip-ring device and a control method thereof in which it is possible to drive loads only by several slip-rings with no limit to the number of loads provided in a rotation device according to a control by a communication method using an electronic control device. The multi-slip-ring device includes first to third power supply slip-rings and first and second communication slip-rings, which are fixed to a rotation shaft to rotate in cooperation with the rotation shaft and are disposed at a predetermined distance, and first to fifth brushes that come in contact with outer circumferential edges of the slip-rings and transmit 3-phase power and control signals to the slip-rings. As described above, to transmit the power to the rotor, it is essential to directly connect the external 3-phase systematic power supply.